I am serving with accounting software sells and implementation firm. My responsibilities are to implement the software and give after sales support.

One of my customer's accountant was fired due to non conformance. That customer asked me to do part time job for him.

Do I legally or ethically bonds not to join him?

If the job does not impact or interfere with your primary job then it would seem you should be free to do so. However, if you will be performing duties similar to those you did for your primary employer, I would not do it.

Either way, I would check with your primary employer first.